Are you a small business selling clothing or shoes online? Startups have turned into big businesses in this industry, including Gilt, Fab, and Rue La La, but one thing they all have to deal with is managing inventory.

These companies all deal with “sized” products, or products that could come in small, medium, or large. And it’s not just clothing and shoes — if your online marketplace sells hats, gloves, or even bed sheets, you need to have the right amount of each size ordered. It sounds like common sense, but Stitch Labs, an inventory management service, notes that inventory costs account for 45-90 percent of all business expenses.

It’s important for online retailers to understand how to stock sizes and how customers buy based on size.

While extra-smalls and extra-extra-larges (XXL) often seem to end up in extra inventory, they still wind up accounting for over five percent of an online merchant’s sales, according to Stitch Labs’ research. The company suggests that retailers move away from the class “1-2-2-1″ inventory purchasing guide. That is, buy one unit of smalls, two of medium and larges, and one of extra large. Instead, it believes that a mix of extra smalls, and extra-extra-largest will help boost sales.

Check out the infographic below for other best practices when ordering your inventory: